1. Field of the Invention
The present invention relates to a hologram recording/reproducing apparatus which is used for an external storage in a computer, an audio-visual information storage, or the like, and using holography, records and/or reproduces a hologram in and/or from a hologram medium.
2. Description of the Background Art
A compact disk (CD) is capable of recording 74-minute music data or 650-MB digital data, using a light source with a wavelength of 780 nm and an optical system including an objective lens with a numerical aperture of 0.45. A digital versatile disk (DVD) is capable of recording an MPEG2-system dynamic image for two and a quarter hours, or 4.7-GB digital data, using a light source with a wavelength of 650 nm and an optical system including an objective lens with a numerical aperture of 0.6.
In recent years, a high-definition dynamic image with a horizontal resolution of 1000 or above has been broadcast, and the performance of personal computers has been increasingly higher. Such a factor has made a greater demand for a high-density and large-capacity optical disk. In response to this, an optical disk unit or the like is offered which includes a light source with a wavelength of approximately 400 nm and an optical system including an objective lens with a numerical aperture of 0.85. In this unit, one side of a disk has a recording capacity beyond 20 GB. Hence, an optical disk unit is provided with a light source having a shorter wavelength and an objective lens having a higher numerical aperture, so that its data recording density can be heightened on a disk.
However, there is a limit to such a short-wavelength of a light source and a high-numerical-aperture of an objective lens. This approach to a high-density recording has nearly reached a dead end. Specifically, within a wavelength range of 400 nm or below, a glass material used for a lens disperses wavelengths more widely. This makes it difficult to control its aberration. In addition, the art of a solid immersion lens has been developed for the purpose of realizing a higher numerical aperture. In this art, a lens working distance, or the distance between a lens and a disk, becomes extremely short (approximately 50 nm). This causes the problem of making it harder to exchange disks or such with one another.
Therefore, in order to solve those problems and enhance the recording density of data on a disk, great attention has been paid to a holographic recording art. For example, FIG. 9 shows a hologram recording/reproducing apparatus provided with the system proposed by Horikome and others. It is a schematic view showing the configuration of an optical system of this hologram recording/reproducing apparatus (e.g., refer to Japanese Patent No. 3652340).
The hologram recording/reproducing apparatus shown in FIG. 9 includes a laser light source 201, an isolator 209, a collimating optical system 207, a polarization beam splitter 208, a spatial light modulator 202, a Fourier transform lens 203, a quarter-wave plate 204, and a two-dimensional light-receiving element array 206. A beam of light emitted from the laser light source 201 is collimated by the collimating optical system 207. Then, it is reflected at the polarization beam splitter 208 and is incident upon the spatial light modulator 202.
As can be seen from its plan view in the upper part of FIG. 9, the spatial light modulator 202 is divided into pixels each of which has a width “d”. It turns the polarization direction of light which irradiates each pixel individually according to an input electric signal. Then, it reflects the light. This spatial light modulator 202 is formed, for example, by a liquid-crystal device called a reflection-type LCOS device. This spatial light modulator 202 is divided concentrically into an external reference light area 221 and an internal signal light area 222. In the internal signal light area 222, the polarization direction of light reflected by each pixel is modulated in accordance with a signal to be recorded.
The light reflected at each pixel after its polarization direction has been turned goes straight again through the polarization beam splitter 208. Then, it passes through the quarter-wave plate 204. Thereafter, it is converged upon a hologram disk 205 by means of the Fourier transform lens 203 and turns into signal light. In contrast, the light reflected at each pixel whose polarization direction remains unturned is reflected again by the polarization beam splitter 208. Then, it returns toward the laser light source 201 and is absorbed by the isolator 209.
On the other hand, in the reference light area 221, the polarization direction of reflected light from each pixel is turned according to a specific pattern. The light which has passed through the polarization beam splitter 208 is guided to the hologram disk 205 and turns into reference light.
The hologram disk 205 is formed by sandwiching a hologram recording material 251 between transparent substrates. An interference fringe produced by the intersecting reference light and signal light is recorded as a hologram in the hologram recording material 251. The hologram recording material 251 is made, for example, of a photopolymer with a photo-curing property. The refractive index of this photopolymer is varied and fixed according to the fine intensity distribution of the interference fringe, so that the hologram can be recorded. Every time a hologram is recorded, the hologram disk 205 is rotated to record hologram rows one after another.
When reproducing a signal, the light of the signal light area 222 of the spatial light modulator 202 is cut off, and only the light modulated in the reference light area 221 irradiates the hologram disk 205. The produced diffracted light is detected by the two-dimensional light-receiving element array 206.
This system has the following characteristic. At the time of recording and reproduction, reference light forms a minute speckle pattern on the hologram disk 205. This makes it possible to record a large number of holograms in slightly different positions on the hologram disk 205. On the hologram plane (the focal plane of the Fourier transform lens 203), reference light has a light-intensity distribution obtained by giving a Fourier transformation to the distribution pattern of light on the plane of the spatial light modulator 202. On the spatial light modulator 202, the light is modulated in a random pattern, and thus, a random speckle pattern is formed on the hologram plane alike. When reproducing a signal, reproduction light is not produced until a speckle pattern of reference light at the recording time coincides with a speckle pattern of reference light at the reproduction time.
At this time, if the positional shift of the recorded hologram from the reference light at the reproduction time becomes a speckle size or greater, the quantity of reproduction signal light is conspicuously reduced. In practice, the speckle size is λ/2NA, if the numerical aperture of the Fourier transform lens 203 is NA and the wavelength of light is λ. For example, if the numerical aperture of the Fourier transform lens 203 is 0.6 and the wavelength of the laser light source 201 is 400 nm, then the speckle size is 0.33 microns. This helps record different holograms in positions apart in micron order from each other.
Another condition for selecting and reproducing a hologram as steeply as described above is to thicken the hologram disk 205 sufficiently. As described earlier, the positional shift of the reference light at the recording and reproduction times decreases the signal-light quantity. This is because diffracted light from different parts in the thickness directions of the hologram disk 205 interferes and negates each other. If the hologram disk 205 is not thick enough, adjacent holograms need to be recorded at a longer interval, thus hindering enlargement of the recording capacity.
The conventional hologram recording optical system of FIG. 9 has the problem of recording only a part of an interference fringe in the thickness directions of a thick recording medium which prevents the recording capacity from being increased. FIG. 10 shows a schematic configuration of a recording beam inside of the hologram disk 205 in the optical system of FIG. 9.
As shown in FIG. 10, the light from the pixels of the signal light area 222 and the reference light area 221 spreads through a diffraction phenomenon caused by the pixel apertures. Then, it turns into a parallel beam having a finite width and interferes with each other in the hologram recording material 251. In FIG. 10, the area of an interference fringe is shown by a slashed triangle having a lateral width of 0.4 mm and a height of 0.47 mm in the thickness directions. These numeric values are obtained when the wavelength of the laser light source 201 is 400 nm, the focal length of the Fourier transform lens 203 is 10 mm and the pixel width d of the spatial light modulator 202 is 10 microns.
As can be seen from FIG. 9 and FIG. 10, even if a thick hologram medium (the hologram recording material 251) is employed in the optical system having the configuration of FIG. 9, then the interference fringe is limited to the vicinity of the focal plane of the Fourier transform lens 203. This makes it impossible to secure a practically-effective hologram-medium thickness as well as a sufficient positional selectivity. This causes the problem of hindering enlargement of the recording capacity.